Hydraulic modelling of horizontal-subsurface flow constructed wetlands: Influence of operation time and plant species

Hydraulic behaviour is a very important aspect in the design of a constructed wetland (CW). Different hydraulic models have been widely applied to obtain a better understanding of CW flow properties and provide tools that optimise the design of constructed wetlands as wastewater treatments. This work studied the effects of the time of operation and the plant species used on the hydraulic characteristics of horizontal subsurface-flow (HSSF) CWs. The plug-flow with dispersion (PFD) model, the detention-time gamma-distribution (DTGD) model and a newly developed model, the multi-flow detention-time gamma-distribution (MFDTGD) model, were applied for modelling the water flow over different operating periods in five pilot-scale constructed wetlands planted with different plant species: CW1, unplanted; CW2, Phragmites australis; CW3, Lythrum salicaria; CW4, Cladium mariscus; and CW5, Iris pseudacorus. The PFD model did not provide a good fit to the experimental effluent tracer concentrations, while the DTGD model provided a satisfactory fit and the MFDTGD model, a very good fit, enabling the differentiation of several flow pathways. When using the MFDTGD model, three different types of pathways could be observed: a 'nominal' pathway, a 'tail' pathway and a 'preferential' pathway. However, the high number of parameters needed by the MFDTGD model reduces the validity of this model for use as a design tool. The values obtained for the parameters of each model were in accordance with those in previously reported studies. Regarding the effects of the operation time and the plant species, as time increased, the N-values tended to increase, i.e. the systems tended to behave increasingly like an ideal plug-flow reactor, especially in CWs with more developed plant species (Iris pseudacorus in CW5 and Lythrum salicaria in CW3). Globally, the CWs tended towards more homogeneous distributions of the flow, probably due to biofilm growth and plant root development.